It is known that magnetic, electric, and optical properties of metal oxide nanoparticles depend on their size and shape (Dai, Z. R. Adv. Func. Mater. 2003, vol. 19, p. 9). Based on the above characteristics, it is predicted that the metal oxide nanoparticles are capable of being applied to various fields, such as magnetic resonance imaging contrast media, record storage media, catalysts, energy storage, sensors, and ferrofluid (Zarur, A. J. Nature, 2000, vol. 403, p. 65; Majetich, S. A. Science, 1999, vol. 284, p. 470).
Nanoparticles have been produced through various synthesis methods, which include thermal decomposition of an organometallic precursor, decomposition using an ultrasonic method, reduction of metal ions at high temperatures, or reduction in inverse micelles. Of them, the most frequently used method is one in which a solution containing surfactants is heated to a high temperature, a precursor is added thereto for a short time to form uniform crystalline nuclei, and the temperature is reduced so as to prevent the formation of additional nuclei and to make the growth of the particles uniform. Additionally, various related technologies have been developed.
Korean Pat. Laid-Open Publication No. 2003-0082395 discloses a method of synthesizing uniform spherical metal oxide nanoparticles, in which a metal precursor and a surfactant react in a solvent at 30-200° C. to produce a metal-surfactant complex where the surfactant is bonded to the metal, the metal-surfactant complex is decomposed at 50-500° C. by heating to synthesize uniform spherical metal nanoparticles, and the synthesized spherical metal nanoparticles are separated and then oxidized using an oxidizing agent.
Korean Pat. Laid Open Publication No. 2003-008234, U.S. Pat. No. 6,262,129, and Shouheng Sun, J. Am. Chem. Soc., 2002, vol. 124, p. 8204 disclose a method of producing a magnetite (Fe3O4) nanoparticle material, in which iron salts, alcohol, organic acid, and organic amine are mixed and heated at 200-360° C. In the above method, the particle size is controlled by changing a ratio of iron salts to acid/amine or by coating small nanoparticles with additional iron oxide.
Another conventional technology is a method of producing metal or magnetic nanoparticles, in which, after a metal precursor is synthesized, the precursor is rapidly added to a hot solution where a surfactant is mixed with a solvent (Peng, X. Chem. Mater. 2004, vol. 16, p. 3931).
Meanwhile, synthesizing methods, which aim to control the shape and the size of metal oxide nanoparticles, are known.
For example, a method of producing metal oxide nanoparticles having various shapes, such as sphere, rod, or bullet shapes, is suggested, in which a solution, containing metal, alkoxide, and a surfactant, is heated, and then a metal halogen compound is rapidly added thereto at high temperatures to produce the nanoparticles in a kinetically stable state while the type and the concentration of the surfactant are controlled (Cheon, J. Am. Chem. Soc. 2003, vol. 125, p. 15981).
Another example is a method of producing anisotropic metal oxide nanoparticles at high temperatures by mixing a metal precursor with a surfactant, a solvent, and an oxidizing agent (Park, J. T., J. Am. Chem. Soc. 2003, vol. 125, p. 3408).
However, the production of the metal oxide nanoparticles according to the above methods has the following problems. In Korean Pat. Laid-Open Publication No. 2003-0082395 and Park, J. T. J. Am. Chem. Soc. 2003, vol. 125, p. 3408, it is necessary to conduct an oxidation process of converting the metal nanoparticles into the metal oxide nanoparticles using an oxidizing agent after the metal nanoparticles have been produced. The above two patents are problematic in that reaction efficiency is reduced because a production procedure is complicated due to the two-step process as described above, and a production cost of the oxide nanoparticles is increased because many types of reactant are added U.S. Pat. No. 6,262,129, Korean Pat. Laid-Open Publication No. 2003-0082394, and Shouheng Sun, J. Am. Chem. Soc., 2002, vol. 124, p. 8204 have a disadvantage in that since it is possible to produce the nanoparticles only when using all of three reactants comprising polyalcohol as a reducing agent, organic acid, and organic amine, many types of reactant are used. Particularly, in the course of producing ferrite magnetic oxide containing manganese or cobalt, the oxidation number of iron is 3, and the oxidation numbers of manganese and cobalt are both 2. However, if polyalcohol acting as the reducing agent reduces iron, it is impossible to form ferrite nanoparticles, and chemical equivalence ratios of metals are not in accord with each other, thus the structure of the nanoparticle does not coincide with an inverse spinel structure. Furthermore, due to polyaldehyde and polyorganic acid, caused by a side reaction of polyalcohol as the reducing agent, the action of organic acid as the surfactant is suppressed and a process of separating byproducts is complicated. Peng, X., Chem. Mater. 2004, vol. 16, p. 3931 discloses a method in which after the metal precursor has been synthesized and purified, the metal precursor is rapidly injected to a reaction solution. In the method, since a complicated process of synthesizing and purifying the meal precursor must be implemented and reactants are mixed by rapid injection, it is difficult to synthesize uniform nanoparticles in great quantity. Likewise, Cheon, J. Am. Chem. Soc. 2003, vol. 125 p. 15981 is problematic in that since the metal precursor is rapidly injected into the reaction solution at high temperatures to produce the anisotropic metal oxide nanoparticles, it is difficult to produce the uniform nanoparticles in great quantity, and a complicated process must be conducted in order to precisely control the reaction.
Therefore, the present inventors have conducted extensive studies, resulting in the findings that it is possible to produce magnetic or metal oxide nanoparticles through a single process by mixing a metal precursor, a solvent and a surfactant while heating them without an oxidizing or reducing agent, and that it is possible to produce magnetic or metal oxide nanoparticles having a desired size by controlling the concentration of the precursor or the type of the surfactant, thereby accomplishing the present invention.